METR 104:
Our Dynamic Weather
(Lecture w/Lab)
General Information
about Weather Radars
Dr. Dave Dempsey
Dept. of Geosciences
SFSU, Spring 2011

A weather radar emits a narrow beam of microwave radiation into the atmosphere and records any of this radiation that reflects back to the radar from objects in the atmosphere.

The microwave radiation emitted by a weather radar typically has a wavelength in the range between a few centimeters and 10 centimeters. Radiation with these wavelengths will reflect from objects that are sufficiently large, but will propagate right past smaller objects. Objects typically large enough to reflect microwaves emitted by a weather radar include:

Note that clouds are not on this list. Clouds are aggregates of vast numbers of tiny liquid water droplets and/or ice crystals suspended in the air. Under the right circumstances, cloud droplets and/or ice crystals can coalesce to form rain drops, snow flakes, hail stones, or other precipitation particles. It takes typically about a million cloud particles to make one precipitation particle with a diameter a hundred times larger than the cloud particles. Precipitation particles are big enough to fall out of a cloud and reach the earth's surface before evaporating, whereas cloud particles aren't. In addition, precipitation particles are big enough to reflect microwaves, whereas cloud particles aren't. Hence, weather radars can detect precipitation but can't see clouds at all. (This makes weather-radar maps excellent complements to weather-satellite images, which can show cloud tops but not precipitation.)

A weather radar can typically see no farther than a couple of hundred miles, so the National Weather Service has established a network of radars spaced several hundred miles apart across the U.S. (see Figure 1) to reveal large-scale patterns of precipitation. The information gathered by the network of weather radars is sent to the National Centers for Environmental Prediction (NCEP) in Washington, D.C., which assembles the data into a composite radar summary (see example, see the accompanying weather radar map for the U.S.). (Sometimes these kinds of maps show areas outlined in parallelograms. These are areas where the National Weather Service has issued tornado "watches" or severe thunderstorm watches. A "watch" means that the NWS believes that conditions are favorable for the formation of tornadoes or severe thunderstorms in the very near future.)

In the 1990s the National Weather Service largely replaced its network of older "conventional" radars with about 135 WSR-88D Doppler radars. Like conventional radars, Doppler radars can detect the presence and intensity of precipitation, but unlike conventional radars, Doppler radars can also measure the speed with which precipitation moves toward or away from the radar, which provides indirect information about wind speeds and wind patterns in precipitating clouds. Such information can reveal the presence of strong wind shear near the ground (a hazard when aircraft try to take off or land), as well as circulation patterns (called mesocyclones) that can develop inside some severe thunderstorms before such storms produce tornadoes. (Note, however, that maps constructed from the NCEP composite radar data, such as the accompanying weather radar map, show only precipitation patterns, intensities, etc., not wind speeds and wind directions.)

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